Not Applicable
1. Technical Field
This invention relates generally to holographic diffusers and more specifically to volume holographic diffusers.
2. Background Art
Holographic diffusers are well known in the art. Additionally, LCD displays, which make use of holographic diffusers, are also well known in the art. Typically, an LCD display uses a holographic diffuser either to augment the back lighting of the LCD display or to direct the transmitted display light to an observer located within a particular range of viewing angles. To accomplish this the holographic diffuser directs the diffused light in particular paths of propagation designed to fill a specific range of viewing angles.
For example, if an aircraft cockpit display has a holographic diffuser, the head box of the pilot will be the area that could be occupied by the pilot""s eyes from which the pilot can be expected to view the output of the display. Therefore it is advantageous to design the holographic diffuser to direct the light transmitted by the LCD display to the head box of the pilot. Thus, it is known to redirect light using holographic diffusers.
However, it is difficult to maintain uniform luminance over the desired range of viewing angles and to produce a sharp luminance fall-off at the edges of the viewing angle range. This difficulty exists because each holographic diffuser design causes display luminance to be a variable function of viewing angle. As a result, display luminance can vary detrimentally when viewed from within the pilot""s head box and the luminance cut-off at the fringes of view lacks sharpness. This is generally attributable to two undesirable properties of known holographic diffusers. Firstly, as the light""s angle of incidence on a holographic diffuser approaches the limits of acceptable angles of incidence consistent with its design, the hologram""s diffusion properties begin to break down and the incident light begins to transmit through the hologram without becoming diffused or deviated in propagation angle. Secondly, the corresponding plot of display luminance as a function of viewing angle resembles a bell-shaped curve. This causes the viewed display images to become dim as viewing angles approach the edges of the viewing angle range. Further, considerable wasted light falls outside the useful range of viewing angles owing to lack of sharpness in luminance fall-off at the fringes of the viewing angle range of interest.
FIGS. 1 and 2 show a side view of a conventional diffusion screen arrangement, such as those known in the art. With reference to FIG. 1, a collimated, or partially collimated, white light input beam 10 illuminates a refractive medium substrate 12 and a volume holographic film diffuser 13 at an incidence angle xcex8 with respect to a line normal to the surface of substrate 12. The holographic film 13 diffuses the input light and forms projected output beam 14 over angular range xcex. This angle xcex shown in FIGS. 1 and 2 is the full width angle of the luminance angular distribution profile between amplitude halfpeaks 20, as shown in FIG. 3. Little, or no, color dispersion is noticeable by a viewer.
It is noteworthy that when a collimated, or partially collimated, white light input beam 10 is incident on refractive medium substrate 12 at an incidence angle xcex8 greater or less than 90xc2x0, the beam exiting the volume hologram can be designed to maintain the same (or nearly the same) diffusion angle, xcex, as that for normal incidence.
Alternatively, referring to FIG. 2, with normal incidence of collimated, or partially collimated, white light input, an output beam with a diffusion angle, xcex, can be projected in a direction xcfx86 that is not normal to the substrate. This can improve the luminance of an aircraft cockpit instrument display located below the pilot eye level, and with the instrument display face normal at a considerable (20xc2x0 to 30xc2x0 or more) angle to the pilot""s direct view line. This can be accomplished by projecting the diffused output beam away from the instrument face normal and toward the center of the pilot""s head box.
Also, designs of volume holographic diffusers are possible in which the input white light collimated, or partially collimated, beam and the propagation direction of the diffused output beam both deviate from the substrate (or instrument display face) normal, such as in U.S. Pat. No. 5,280,371 (McCartney). In the field of automotive exterior lighting, there are volume diffuser designs containing certain geometric arrangements of holographic cells for projecting light in a predetermined pattern, such as the vehicle stoplight in EP-A-0 635 651 (Hughes Aircraft Corporation).
In these prior art diffuser designs, the gradual luminance fall-off at the fringes of the viewing angle range (and at angles beyond those fringes) causes a waste of light resulting in reduced display luminance. Therefore, to minimize wasted light and maximize the light flux captured within the viewing angle range of interest, it is advantageous to maximize the slope at the halfpeak points of the luminance angular distribution curve.
This invention is particularly useful as a beam deflecting diffusion screen for displays, such as LCD instrument panel modules in aircraft cockpits and heads-up displays although its application is not limited to displays. A set of narrow superimposed deflected diffused beam profiles with sharp luminance cut-offs at their halfpeak full width points forms a composite angular luminance distribution. By concentrating these superimposed light beams that project from a display panel and by capturing them within the pilot""s head box, efficiency is improved by minimizing the light wasted by projection outside the pilot""s head box. Although an individual projected narrow beam angular profile does not, by itself, render the display luminance uniform as a function of viewing angle, the superposition of a plurality of individual narrow beams can be designed to generate uniform luminance over a wide viewing angle range of interest.
The invention is accomplished with the structure and method of the present invention by sending collimated, or partially collimated, light through a substrate with a film matrix comprising a nested plurality of individual joined geometrically shaped holographic cells. The cells comprising the matrix are subdivided into groups. Each cell within a group contains a uniquely patterned volume holographic diffuser. This generates a diffused narrow light beam projected in a direction diverse from that projected by every other cell in the group. The superposition of the variously directed diffused narrow light beams projected from each cell group produces a combined resultant diffused wide light beam. The resultant light beam has a luminance angular distribution profile with sharply vertical slopes at its halfpeak points and a substantially flat and wide peak over a wide viewing angle range of interest. The display luminance thus produced is uniform over a wide range of viewing angles. This range of angles is centered on a specific beam deflection angle that passes through the midpoint of the pilot""s head box. Thus the matrix of cells on the display produces a uniform luminance over the entire surface of that display and when viewed from any point within the pilot""s head box. The resulting display luminance is substantially uniform over a wider range of viewing angles than is known in the art and the sharpness of the luminance fall-off at the angular distribution profile halfpeak points is greater than is known in the art. In the preferred embodiment, a non-alternating, single image is also provided for both eyes rather than alternating a separate right eye image with a separate left eye image.
This invention is most useful for applications where collimated, or partially collimated, light is incident on a display and a need exists to project the light transmitted by the display into a wider and more diffuse beam. A further enhancement of its usefulness occurs when the projected diffuse beam is uniformly distributed over a desired wide range of viewing angles and with sharp luminance cut-offs at the edges of that range. The projected diffuse beam can also have an asymmetric (i.e. different angular widths in orthogonal profile planes) output beam envelope, which has a high efficiency and little or no color dispersion. It may also be desired to have the option of deflecting the axis of this output beam envelope at a different angle from the input beam direction. An asymmetric output beam envelope and/or one having an axis different from that of the input beam is useful for minimizing light flux that fails to fall within a pilot head box having asymmetric dimensions and/or one that is positioned away from the display normal.
By creating a matrix of volume holographic cells arranged in a regular pattern on or within the surface of the diffusion screen, the adjacent cells of a subgroup of the matrix have different holographic designs each of which deflects the diffused beam projected therefrom in a different direction. The beam spread and deflection direction of each projected output beam can be controlled by means of each different subgroup cell holographic design. The superposition of diffused projected output beams thus produced generates a composite angular luminance distribution with sharp profile slopes at its halfpeak points and a substantially flat wide peak. The composite projected beam has the desired diffusion spread and propagation direction.
Thus, the present invention uses a method and apparatus for sending light beams from a display through a substrate matrix of nested individually joined geometrically shaped cells. The cells are divided into subgroups wherein each cell of a subgroup contains a patterned volume holographic diffuser with a different design or projection angle for optimal diffusion to occur. Each cell of a subgroup projects a diffused light beam with a different angle of propagation from that of the other cells of the subgroup.
Owing to the holographic diffuser""s repetitive pattern of cell subgroups, there are many more cells than beam projection directions. Therefore each cell has a beam projection direction shared with many other cells in the matrix. The angular distribution of light incident on a holographic diffuser cell can be widened by the cell""s diffusion properties. Thus the angular distribution of the beam projected from that cell can be wider than that of the incident light beam. Further, the beam projected from that cell can propagate in directions diverse from that of other cells of its cell subgroup. Therefore the angular distribution of the composite beam projected from a subgroup of cells can be wider and, possibly more angularly asymmetric, than any of the individual component beams comprising the composite beam. Further, because the composite beam can be comprised of a plurality of individual beams having narrow angular distributions (compared with the composite beam""s distribution), the angular distribution profile slope at the composite beam""s halfpeak points can be sharp and nearly vertical, similar to that of the narrow beams. When the display is viewed from points in the pilot""s head box, display luminance can be a uniform function of viewing angle because the peak composite projected beam""s angular distribution is substantially flat over a wide range of viewing angles. Thus a predetermined beam spread and deflection angle is created in relation to the viewer. Photometric efficiency is maximized by virtue of high, nearly vertical, slope angles produced at the fringes of the luminance angular distribution profiles projected from cell subgroups across the display surface.